The epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein encoded by the c-erbB1 proto-oncogene. Sequence analysis demonstrated that the human EGFR is the cellular homolog of the v-erbB oncogene from the avian erythroblastosis retrovirus (Downward et al., Nature, 307, 521 (1984); Ullrich et al. Nature, 309, 418 (1984)). In recent years, a series of c-erbB related cell surface receptor tyrosine kinases has been identified. The four members of the c-erbB proto-oncogene family are: ErbB1/EGFR, ErbB2/Neu (Coussens et al., Science, 230, 1132 (1988)); ErbB3 (Kraus et al., Proc. Nat'l. Acad. Sci. USA, 86, 9193 (1989)); Plowman, et al., Proc. Nat'l. Acad. Sci. USA 87, 4905 (1990)); and ErbB4 (Plowman et al., Proc. Nat'l. Acad. Sci. USA, 90, 1746 (1993)). Distinct genes encode these receptors.
The ErbB1/EGFR includes three functional domains: an extracellular ligand binding domain, a transmembrane domain, and a cytoplasmic tyrosine kinase domain The extracellular domain can be further divided into four subdomains (I-IV), including two cysteine-rich regions (II and IV) and two flanking regions (I and III) (Lax et al., Cell Regul. 2, 337 (1991)). Subdomains I and III are involved in ligand binding. Ligand binding to the receptor represents the first event in a complex cascade culminating in DNA synthesis and cell division. The full-length 170 kD human EGFR is encoded by two alternatively spliced transcripts of 5.8 and 10.5 kb (Ullrich et al., supra). A truncated 80 kD EGFR present in human placenta extracts may be encoded by an alternatively spliced 1.8 kb RNA transcript (Ilekis et al., Mol. Reprod. Devel., 41, 149 (1995)). Furthermore, soluble EGF receptors arise from aberrant transcription products in carcinoma derived cell lines, as exemplified by the epidermoid carcinoma line, A431 (Ullrich et al., supra). In this cell line, the EGFR gene is amplified and rearranged, and a 2.8 kb transcript arises from a translocation between the 5′-region of the EGFR gene and an unidentified region of genomic DNA (Ullrich et al., supra; Merlino et al., Mol. Cell. Biol., 5, 1722 (1985); Hunts et al., Somat. Cell Mol. Genet., 11, 477 (1988)). Alternatively spliced c-erbB1 RNA transcripts of approximately 2.6-2.7 kb that encode secreted, truncated receptors containing only the extracellular ligand binding domain are also found in normal chicken and rat tissue (Maihle et al., supra; Petch et al., Mol. Cell. Biol., 10, 2973 (1990)).
Soluble forms of the ErbB family of receptors have been investigated in connection with several cancers (McKenzie, Biochim. Biophys. Acta, 1072, 193 (1991); Brandt-Rauf, Mutat. Res., 333, 203 (1995)). Immunoassay studies show that sErbB2 proteins are elevated in serum samples of patients with breast and ovarian cancer (Mori et al., Jpn. J. Cancer. Res., 81, 489 (1990); Meden et al., Anticancer Res., 17, 757 (1997)). Recent studies suggest that low pretreatment serum sErbB2 levels are positive predictors of responsiveness to hormonal therapy for patients with metastatic breast cancer (Hayes et al., Breast Cancer Treat., 14, 135, (1993); Leitzel et al., J. Clin. Oncol., 13, 1129 (1995); Yamauchi et al., J. Clin. Oncol., 15, 2518 (1997)). Meden et al. (supra) have reported a positive association between elevated serum p105 sErbB2 levels and shorter survival for patients with stage I through IV epithelial ovarian cancer (EOC). In addition, U.S. Pat. No. 5,674,753, issued Oct. 7, 1997, described the use of antibodies against the external EGF binding domain of EGF receptors to diagnose neoplastic diseases correlated with an increase in the level of an EGF receptor in a patient's blood.
Immunoassay studies show that the extracellular domain of ErbB 1 is detectable and increased in the serum of patients with asbestosis-induced lung cancer (Partanen et al., J. Occup. Med., 36, 1324 (1994); Partanen et al., Int. J. Oncol., 4, 1025 (1994)) and in the urine of patients with squamous cell carcinomas of the head, neck, and lung (Witters et al., Clin. Cancer Res., 1, 551 (1995)). Ilekis et al. (Gynecol. Oncol., 65, 36 (1997)) have recently observed a positive association between levels of a p80 sErbB1 protein and full-length ErbB1 in tissue samples of serous cystadenocarcinomas of the ovary. In addition, ErbB1 receptors have been shown to be over-expressed in various human tumor cell lines and neoplasms (Xu et al., Proc. Nat'l. Acad. Sci, 81, 7308 (1984); Salomon et al., Crit. Rev. Oncol. Hematol., 19, 183 (1995)), including cancers of the breast (King et al., Science, 229, 974 (1985)), lung (Hendler et al., Proc. Am. Soc. Clin. Oncol., 8, 223 (1989); Veale et al., Cancer Res., 49, 1313 (1989)), brain (Schlegel et al., Int. J. Cancer, 56, 72 (1994)), bladder (Neal et al., Cancer, 65, 1619 (1990); Mellon et al., J. Urol., 153, 919 (1995)), and ovary (Berchuck et al., Am. J. Obstet. Gynecol., 164, 669 (1991); Scambia et al., J. Clin. Oncol., 10, 529 (1992)).
Thus, the current research concerning the biological role and function of EGF receptors has been contradictory, and does not provide a clear indication of how any particular EGF receptor can be used as a tool in diagnosing any particular cancer type. Therefore, a need exists for the isolation of soluble epidermal growth factor receptor protein molecules and their isoforms, and for the characterization of their correlated disease conditions. In addition, a useful, quantitative diagnostic method to detect the presence of biologically relevant isoforms for diagnosing the onset and progression of diseases associated with these soluble epidermal growth factor receptor protein molecules is also needed.